The present invention relates to a field emission device that produces light of selected wavelengths ranging from ultraviolet to infrared.
In recent years, there have been high expectations for the introduction of new types of lamps. Lamps emitting light in specific wavelength ranges are required in several different fields of use. For example, cold white light is desirable for car interior lamps, decorative office lamps, hospital lighting such as surgery lighting, and for Liquid Crystal Display (LCD) backlighting. Infrared (IR) light can be used in security, military and other night vision applications. Ultraviolet light can be used for skin tanning, counterfeit detection, exciting a photo-catalytic material, polymer curing, purification, and for sanitation.
White light lamps can emit a broad range of light with a wavelength from 400 nm to 700 nm. Although several lighting techniques have been developed to produce white light, such as fluorescent illumination, incandescent lighting, electro-luminescence and Light-Emitting Diodes (LED), those techniques have disadvantages.
Incandescent lamps generate visible light using a hot filament. The efficiency of incandescent lamps using filaments has not increased significantly for many years. When light is generated from an incandescent lamp, the filament generates significant heat through infrared radiation. Therefore, incandescent lamps require high power dissipation, are relatively inefficient and rapidly increase the temperature of the surrounding environment. Another problem of incandescent lamps is non-uniformity of illumination. It is hard to produce planar luminescence light.
Fluorescent lamps are operated by connecting a high voltage to two electrodes. Plasma, which is generated from the open discharge, creates Ultra-Violet (UV) radiation from mercury vapor in the lamp. Visible light is excited by the UV radiation from a phosphor coating on the inner surface of the lamp. Because of the mercury, fluorescent lamps cause pollution problems during manufacturing and recycling. It is also difficult to produce planar luminescence light.
Electro-Luminescence (EL) lamps suffer from low brightness and a short lifetime. Compared with incandescent lamps and fluorescent lamps, EL lamps have markedly lower brightness. Moreover, the brightness of EL lamps decreases over time. Typically, the brightness of an EL lamp decays about 50% after 1500 hours.
Light-Emitting Diodes (LED) cannot be flat. Additionally, the power consumption of LED's is comparable to or greater than electro-luminescence lamps and fluorescent lamps. Accordingly, it would be desirable to produce a cold white light lamp with low power dissipation, high brightness, and low heat generation to remedy the defects of the prior art.
For security applications, a near infrared spotlight is an ideal light source for a Charge Coupled Device (CCD) camera. Normally, the CCD camera has a sensitive range from 400-1000 nm. With a near-IR spotlight glow, near infrared light (from 750 to 930 nm) is generated. While near-IR light is a suitable light source for a CCD camera, near IR-light is not detectable by human eyes. Thus, such a lamp has broad application in security, military and other night vision fields.
A conventional near-IR spotlight has an incandescent lamp and an IR pass filter to create near-infrared light. However, the large amount of heat generated by the hot filament in the incandescent lamp leads to a short lifetime of the filter. There is a demand for lamps that produce infrared radiation without visible light or heat.
Conventional fluorescent lamps, incandescent lamps, and LED lamps can generate UV light, but have the drawbacks discussed above. Therefore, there exists a need for improved lamps that can produce desired UV light without visible light or IR light.
Field emission lamps are known. An exemplary structure of a field emission lamp is disclosed in U.S. Pat. No. 6,111,354, the contents of which are hereby incorporated herein by reference. A phosphor-coated anode and a field emission cathode are enclosed together in a panel. A voltage source is connected to the cathode and anode in a diode structure; or connected to the cathode, gate and anode in a triode structure. The field emission electrons are accelerated from cathode to anode. When electrons impact a phosphor layer on the anode, the phosphor is excited to luminescence. Field emission lamps are lightweight and have a high luminous efficiency, low power consumption, long lifetime, low working temperature, large panel size and planar structure.
However, a light source with a selective wavelength is desirable to improve upon the prior art.